Nanoparticle-host interactions in one, two and three dimensions: biomineralisation

Biomineralization is a complicated interplay between organic molecules or so-called organic templates and single ions or small aggregates of cations and anions that form the nucleus of biominerals growth. In the case of interactions with single ions in solution, organic molecules act as complexing agents to transport the ions that finally form the biomineral to the actual growth site. Often, these ions cannot be transported as free ions because uncontrolled precipitation may occur before the target growth site is reached. Once the growth process has started, interactions between the nanoclusters forming the initial growth nucleus and organic molecules control the shape and growth kinetics of the growth island. These organic molecules can be oligomers or polymers that stabilize certain step directions; thus, the nanoparticle-host interaction is one-dimensional.

Both organic molecules that adsorb to growth nuclei and an organic template determine the morphology of the biominerals, which can be highly elaborate as in the case of coccolithophores. These two-dimensional growth island(nanoparticle)-host(template) interactions are stronger, when the interface energy (per unit surface area) between the organic template and the growth island is more negative. This scenario is obtained if, e.g., the negative functional groups of the organic template have a commensurate periodicity as the arrangement of the calcium ions on a particular calcite surface that is in contact with the template. Typically, we consider crystal faces to be stable if their charge is neutral and if their dipole moment perpendicular to the surface is zero as these conditions often lead to stable surfaces with low surface energies. The situation can be different in biomineralization processes: the organic template can expose an array of negative functional groups such as deprotonated carboxylic groups or alternating positive and negative functional groups as in amino acids. In the first case, the strongest interface would be formed if the calcite surface is positively charged. Thus, a calcite (001) surface with Ca termination may be a good candidate for such an interface even though calcite (001) surfaces are usually not found in inorganically grown calcite crystals because their charged surface results in a high surface energy. Not only must the polarity of the organic template and a particular crystal surface match but so must the distance between them and the angle between chains of functional groups and the respective angles on the mineral surface. One method to gain control of the periodicity in a laboratory experiment is the application of Langmuir-Blodgett films. Organic molecules with a hydrophilic group on top of a water surface and a lipohilic group above can be more or less squeezed together using a system of micropistons. Using this method, Buijnsters et al. (2001) varied the periodicity and chemical nature of the functional groups. As a result, in the supersaturated calcium carbonate solution underneath the film, they observed the growth of different faces of calcite, aragonite, and even the metastable vaterite as a function of separation between the hydrophilic groups of their Langmuir film. Another crystal growth pathway involves the formation of amorphous calcium carbonate (ACC) as a precursor during morphogenesis (e.g., Wei et al., 2007).

These model systems are helpful to understand the overall mechanism of biomineralization. However, the exact nature of the exopolymer, the organic template on the exoskeleton that organisms use to stabilize a specific carbonate surface, is still unknown. Therefore, we describe how different polypeptide sequences interact with calcite surfaces and steps (one-dimensional interaction) and how the interaction in Langmuir Blodgett films can be simulated at an atomistic level.

Adsorption of oligomers to step edges as a precursor of biomineralization

As a precursor of biomineralization, we are studying the interaction of oligomers/polypeptide chains on the (10-14) calcite surface, the lowest energy faces of pure calcite. The main objective is to find suitable orientations of amino-acid residues in these peptide chains, where these peptide chains align themselves parallel to the calcite surface. The stereochemical relation between the coordination environment of ions in specific crystal faces (Ca2+, CO32- in this case) and the arrangement of ligands (that is peptide residues, oligomers) around ions bound to the surface is a potential factor for organic nucleation and selectivity of biominerals. If the distance between the repeating residue units in the adsorbent matches the distance between the repeating units in the adsorbate surface, the adsorbent long-chain polymer or oligomeric organic compound can lie parallel to the surface step. Calculations of various sequences of small chain peptide residues along non-polar (periodic bond chains of alternating Ca2+ and CO32- ions with no dipole moment perpendicular to the step) and on polar surface steps (steps bounded by either Ca2+ or CO32- ions) were performed (Becker et al., 2005; Biswas & Becker, submitted). Each peptide residue was composed of three amino acids. The strongest interaction is obtained for the sequence (phe-leu-lys)4-, where the total adsorption energy to a non-polar calcite surface step is -1.071 eV (average of 0.357 eV/(amino acid unit) ). Adsorption energy values for other peptide chains vary between –0.1879 eV/amino acid to –0.2890 eV/amino acid.

Almost parallel alignment of these 3-amino acid peptide residues is observed when aligned with polar Ca2+-bounded surface steps where the dominant interaction between the negatively charged peptide backbone and Ca2+ along the surface step is of electrostatic nature. Adsorption energy values in this case range between –0.1733 eV/amino acid to –0.2602 eV/amino acid. The most energetically favourable adsorption is obtained for the sequence (phe-leu-lys)4- with -0.2978 eV/amino acid.

Using the same concept, the adsorption of longer peptide chains having 12 amino acids was simulated. Sequencing of amino acids has been obtained from encoded GPA, a calcium binding protein in the coccolithophorid Emiliania Huxleyi (Corstjens et al., 1998). The interaction of this 12-amino acid long peptide chain in both acidic (protonated) and alkaline (deprotonated) conditions with polar calcite steps with Ca2+ and CO3- bounding was studied. At alkaline conditions, the adsorption energy of the negatively charged peptide with the Ca2+-bounded step edge is –0.09824 eV/amino acid. When the peptide residue is neutral (acidic condition), the adsorption energy is -0.1978 eV/(amino acid residue) (CO32- at step edge). In this long chain peptide, amino acid residues at the middle part of the chain are closer to the surface than the end-members giving a U shape to the peptide chain (Fig. 24A). Better parallel inclination of the peptide chain to the calcite surface is observed in the neutral peptide rather than the –13 charged peptide residue. This can be owed to the fact that in the negatively charged long chain peptide, electrostatic interaction between the charged side chain of the amino acid and the peptide backbone coupled with steric hindrance due to the large side chain prevents parallel alignment. It is also known that proteins can inhibit calcite growth and that ability is attributed to backbone flexibility of the peptide chain (Gerbaud et al., 2000). This may be another reason for why we see flexible, not parallel, peptide chains on these polar steps.

These simulations show that the presence of large side chains in the amino acid residues can cause steric hindrance and preventing the peptide chain from aligning parallel to the calcite surface. In the 12-amino acid peptide residue, the presence of a 5-member heterocyclic ring in proline, CH(CH3)-CH2-CH3 in isoleucine, and CH2-CH2-COOH in glutamic acid are responsible for the steric hindrance causing the U shape of the peptide chain. To minimize this steric hindrance, the large side chains in amino acid residue have been replaced by H- and CH3- groups, thus constructing a 12-residue peptide chain with alternating glycine and alanine amino acids. At low pH, this 12–amino acid long glycine-alanine peptide residue aligns itself more or less parallel to the calcite step edge, verifying that large side chains in amino acids are major obstacles in parallel alignment (Fig. 24B). The adsorption energy for this gly-ala peptide residue is –0.0428 eV/(amino acid residue) along the polar CO32- step. The average distance between the amino acid residues in this peptide chain is 3.185 Å.